1. Field of the Invention
The present invention relates to an image sensing imaging device used in an automatic focus detecting system of a photographic camera.
2. Description of the Prior Art
Hitherto, this type of image sensing device is disclosed in, for example, the Japanese Laid-open Patent Publication No. 60-125817. The image sensing device disclosed therein is constructed in the form of a CCD (charge-coupled device) and has an accumulator, a drain and an integration clear gate, all of which are arranged between a plurality of photo-diodes as a photoelectric converter and a transfer register. When unnecessary charge is to be drained from the transfer register at the time of initialization, the frequency of transfer clocks is increased to minimize the time required for the initialization.
According to the Japanese Patent Publication No. 61-17387, there is disclosed an image sensing device wherein, in order to determine the integrating time, both of a gate provided between the accumulator and the transfer register and a gate provided between the transfer register and the drain are opened for a predetermined time so that the unnecessary charge of both of the accumulator and the transfer register can be drained to the drain, but both of said gates are closed during the integration so that the integration can take place only during a period corresponding to the difference between the predetermined time and the time required for the drainage of the unnecessary charge, the first mentioned gate being opened pulse-wise upon the termination of the integration to transfer the charge accumulated in the accumulator to the transfer register.
However, in the first mentioned image sensing device, although the unnecessary charge can be drained from the accumulator through the integration clear gate, the direct drainage of the unnecessary charge of the transfer register is not possible. Therefore, the frequency of the transfer clocks is necessarily increased during the initialization to facilitate the drainage of the unnecessary charge of the transfer register. The increase of the frequency of the transfer clocks brings about such a problem that a drive circuit for generating the transfer clocks may become complicated in structure. Also, in the first mentioned image sensing device, since the integration clear gate and other gates are, in addition to the accumulator, provided between the photo-diode and the transfer register, the intervention of the integration clear gate and the other gates provides an obstruction to the reduction in pitch between each neighboring picture elements.
In the second mentioned image sensing device, although the time required to accomplish the integration is in effect short where a object aimed to be photographed is very bright, the integrating cycle actually taking place in such device is fixed at the predetermined time. Therefore, as compared with the case in which the integrating time varies with brightness of the object, the time required to give an integrating data for each occasion tends to become long. Accordingly, where this integrating data is to be used in the focus detection in the photographic camera, a problem will arise in that the cycle of focus detection does not follow the movement of the object. Moreover, where the object is dark, another problem will arise in that the photoelectrically converted output is low in the case of the predetermined time of the integrating cycle and, therefore, no integrating data effectively utilizable in the subsequent stage can be obtained.